This invention relates to instruments for monitoring the direction of a borehole either continuously or at a series of locations spaced along the length of the borehole, and to methods of surveying a borehole.
A spatial survey of the path of a borehole is usually derived from a series of values of the azimuth angle and the inclination angle taken along the length of the borehole. Measurements from which the values of these two angles can be derived are made at successive locations along the path of the borehole, the distances between adjacent locations being accurately known.
In a borehole in which the earth's magnetic field is unchanged by the presence of the borehole itself, measurements of the components of the earth's gravitational and magnetic fields in the direction of the casing-fixed axes can be used to obtain values for the azimuth angle and the inclination angle, the azimuth angle being measured with respect to an earth-fixed magnetic reference, for example magnetic North. However, in situations in which the earth's magnetic field is modified by the local conditions in a borehole, for example when the borehole is cased with a steel lining, magnetic measurements can no longer be used to determine the azimuth angle relative to an earth-fixed reference. In these circumstances, it is necessary to use a gyroscopic instrument.
Applicants' Great Britain Patent Specification No. 1,509,293 describes such an instrument comprising a casing having its longitudinal axis coincident, in use, with the axis of the borehole, a single-degree-of-freedom gyro comprising an outer gimbal mounted in the casing with its axis coincident with the longitudinal axis thereof, an inner gimbal mounted in the outer gimbal with its axis perpendicular to the outer gimbal axis, a gyro rotor mounted in the inner gimbal, means for sensing angular movement of the inner gimbal relative to the outer gimbal and means for applying a torque to the outer gimbal for rotating it in use about its axis so that the inner gimbal processes back to its initial position, means for measuring the angle of rotation of the casing about its longitudinal axis relative to the outer gimbal and a gravity sensor unit for measuring three components of gravity in three non-coplanar directions.
This instrument has proved to be extremely reliable in practice and has been shown to be capable of an accuracy of up to approximately .+-.0.1.degree. in inclination and .+-.1.0.degree. in azimuth. The stated maximum inclination for such an instrument is generally considered to be about 70.degree. to the vertical since surveying at angles in excess of 60.degree. leads to increasingly less accurate surveys as the inclination increases. However, with the trend towards high inclination drilling, there exists an increasing demand for an instrument with an accuracy in azimuth of the same order of magnitude as that attainable in inclination. It is an object of the invention to provide such an instrument.